Before you start

Keep the serving plates warm on the stove while you're preparing the dish.

Method

In a small bowl, macerate the prunes in the beer for 15-30 min. Drain them, reserve the liquid, and set aside.

Preheat the oven to 160 °C / 325 °F.

Slit the fillets in half lengthwise in order to form a pocket, then salt and pepper the inside. Place 6 prunes side-by-side in each fillet. Close the pocket using tooth-picks (3 per fillet).

Heat the oil and butter in a pan to a fairly high temperature. Brown the fillet(s) on each side, 6-8 min total, then transfer them into an ovenproof dish. Sprinkle with 1-2 tablespoons of the macerating beer, and bake for about 13 min.

Meanwhile, add the shallot to the pan, then sauté 3 min
with stirring. Add the remaining macerating beer and deglaze until the liquid is reduced to a half (2-3 min). Add the sage and the demi-glace, then cook over medium heat for about 5 min.

Take the dish out of the oven and remove the tooth-picks. Transfer the fillets with all the meat juice to the stovetop pan. Cook for 1-2 more min. Add salt and pepper to taste. Slice the fillets then place them on the warmed serving plates. Cover the slices with the cooking juices and serve.